Keep the information of gate duration after the compilation

[BoxiLi]

Currently, the compiler only returns the compiled pulse cofficients and tlist. It is sufficient for the solvers, but a few options in the solver may need more information. For instance, max_step is needed to prevent the solver from skipping pulses after a long idling period or initial states that are invariant under the given Hamiltonian (qutip/qutip#2003, qutip/qutip#2040).

This can be realized in 2 different ways:

• The Compiler returns more information. On top of the compiled pulses, we could let the compiler return also additional information such as the minimum gate time.
• Include the information in e.g. in the Coefficents class of qutip-v5 and return the Coefficient Object. This may also provide the opportunely to support different max step sizes for different gates. But it needs considerably more work and may need changes in the core qutip.